At present there are various types of prior art filter bags for products for infusion, which contain metered quantities of a substance, and which are designed to be immersed in a container holding boiling liquid at the moment of preparing the infusion.
Such filter bags are usually made from a web of filter material which is subjected to a series of successive folds in such a way as to form a tubular element, inside which metered quantities of product are inserted. The tubular element is then cut into pieces of suitable length, folded in half over themselves and closed at their free ends which, by means of a string, are connected to the pickup tag.
Amongst the various techniques for closing the ends of the bag, closing achieved by folding the end over itself then stitching with a knotted thread has been established for some time now with considerable success. Said technique avoids the addition to the infusion liquid of extraneous agents which are not natural, and which can potentially contaminate the infusion in terms of both health and hygiene, and organoleptic properties, since they may alter the flavor of the infusion.
The technique of connecting by knotting, invented for the filter bag, was then also extended to the pickup tag. This was done mainly to have uniform technology, since it would not strictly be essential, considering that the tag is not normally intended to make contact with the infusion.
A critical aspect for all of the prior art solutions involving connection by knotting is obtaining: on one hand, suitable intensity of the force for tightening the connection; and on the other hand, stably maintaining said tightened condition even when the bag remains in infusion in the boiling liquid for a relatively long time.
Indeed, loosening of the knot may prove particularly critical, during immersion of the bag, both due to the possibility of losing control of it if it accidentally becomes disconnected from the string connecting it to the pickup tag, and because of the possibility of causing the dispersion, in the infusion liquid, of the leaves of the essence contained, since the knot connecting the pickup string to the bag often also allows simultaneous sealing of the bag.
Amongst the various systems for packaging with a knotted string, one prior art solution involves basically forming the connection by creating a single loop wound on itself with one or more full turns about its own axis before being passed through by a portion of the string which, together with the loop, contributes to forming a connecting knot.
When the knot is tightened, the torsion previously imparted to the loop and the winding of the string on itself caused by it generate a friction between the various parts of the string which is such that it prevents, in package use conditions, backward movement of the portion of string relative to the loop, thus guaranteeing that the secured condition imparted by the knot is stably maintained.
Making such a knot, whose design is relatively simple, in actual production with high speed automated machinery, proved quite laborious. It is done using machines having rather complex construction which use needles which have a particular shape and special operations, interacting with a set of auxiliary elements whose shape is equally specific and dedicated.
Due to said complexity, such machines are quite expensive. Even the cost of operating these machines is high. The needles, which are in themselves expensive, are subject to rapid deterioration due to wear, meaning that they have to be substituted regularly. The high level of friction created in the string of the knot during tightening is one of the main causes of said rapid wear on the needles.
U.S. patent application 2001/0053400 to Lohrey et al. discloses an infusion bag, especially for tea, which has a top part with folded down corners and a middle part folded over it, to which a string with two free ends is attached that goes from one side of the top part through a hole in the folded down parts forming a loop on the other side of the top part and comes back, so that the two free ends are drawn up through the loop laid over the top edge of the infusion bag and pulled to form a knot. Two holes are made in the top part at a lateral distance next to one another, on the left and right of the longitudinal axis of the bag. Each hole goes through a folded down corner and the middle of the top part, and the string forms a loop and one end goes through one hole and the other end through the second hole. The loop is laid over the top edge of the infusion bag and both ends of the string go up through the loop to form a double knot. This patent application also discloses a method of closing an infusion bag.